In the optical subscriber transmission system, especially in a PON (Passive Optical Network) system in which packets of data signals of each subscriber are time-multiplexed, each subscriber has a different signal transmission distance. For this reason, it is necessary to receive optical signals at different receiving levels in an OLT.
In a receiving circuit, it is required to compensate the difference in the receiving levels, and to generate a signal at a certain level so that a discriminator at a subsequent stage can discriminate and reproduce that signal.
There are conventionally known means for compensating a receiving level difference as follows. One is a method in which a gain of an amplifier is controlled by detecting the level of a received signal. The other is a method in which a difference between the level of a received signal and the center value of amplitude, that is an offset, is compensated by detecting the level of a received signal.
Particularly, in the PON system in which packets of a data signal is transmitted at shorter intervals, fast compensation for a receiving level difference is required. Hence, the latter, offset compensation system, enabling a fast response has been used in the PON system. In other words, offset compensators (AOC) are conventionally used for the purpose of achieving higher receiving sensitivity by canceling the offset.
As shown in FIG. 1, a conventional receiving circuit includes a photodiode (PD) 100, a preamplifier 200B and a post amplifier 300B. The preamplifier 200B functions as a trans impedance core circuit for performing current-voltage conversion by use of an amplifier 201 and a feedback resistor 202. The post amplifier 300B compensates an offset of a data signal by use of an amplifier 301 and an offset compensator 302.
FIG. 2 shows a waveform diagram of basic operations of the receiving circuit.
The preamplifier 200B receives a current signal Iin obtained by converting an optical signal into an electronic signal by the photodiode 100. The preamplifier 200B then converts and amplifies the received current signal (input current signal) Iin. Thereby, an output voltage signal Vout1 (output data signal Data) is generated, and the preamplifier 200B outputs the output voltage signal Vout1.
The preamplifier 200B is not provided with an offset compensator. For this reason, even when the input current signal Iin has a level difference (amplitude difference), the preamplifier 200B generates the output voltage signal Vout1 while maintaining that level.
The post amplifier 300B at a subsequent stage detects the level of the output voltage signal Vout1 and compensates an offset. Hence, the post amplifier 300B outputs an output voltage signal Vout2 having a high-quality waveform without distortion.
Specifically, the post amplifier 300B detects an amplitude difference for each packet of the output data signal Data by use of the offset compensator 302. Thereafter, the post amplifier 300B corrects an offset voltage Vaoc which is a difference between the level “0” and the center value of amplitude. The post amplifier 300B then amplifies the corrected voltage by use of the amplifier 301.
In order to detect the level of each packet of the output data signal Data, an external reset signal RST2 for initializing (resetting) the offset compensator 302 is inputted to the offset compensator 302 just before the packet is received.
However, the above-described conventional receiving circuit has the following problem. When the level of the output voltage signal Vout1 of the preamplifier 200B varies in packets, a level detection error occurs in the offset compensator 302. This leads to deterioration in the offset compensation accuracy, thus causing deterioration in the waveform quality.
Especially, suppose that the value of the feedback resistor 202 is changed to switch a gain as shown in FIG. 3 in order to widen a dynamic range of input to the preamplifier 200B. A problem may occur that a level detection error is caused when a higher gain of the preamplifier 200B is switched to the lower gain. The cause of the level detection error is a notch in the waveform of the output voltage signal Vout1. The notch occurs when there is a circuit delay between the level detection and the gain switching.
FIG. 4 shows I/O characteristics shown in a case where the value of the feedback resistor 202 of the preamplifier 200B is switched (RF1, RF2, . . . RFn). The ratio of the output amplitude (output voltage signal Vout1) to the input current signal Iin is the gain (conversion gain) of the preamplifier 200B. Thus, the larger the inclination of the line indicating the above ratio in FIG. 4 is, the higher the gain is. Meanwhile, a smaller inclination indicates a lower gain.
In addition, the gain reflects the value of the feedback resistor 202. A higher resistance is used to obtain a higher gain, and a lower resistance is used to obtain a lower gain.
FIG. 5 shows examples of operation waveforms in a case where the value of the feedback resistor 200 is changed.
In a case where the preamplifier 200B is waiting for the incoming of the input current signal Iin with the maximum gain (with the resistance value RF1 of the feedback resistor 202), the maximum gain is switched to smaller one when the input current signal Iin is sufficiently large (the resistance value RF1 switches to RF2).
At this stage, due to a circuit delay caused by a switch control circuit (not illustrated), a delay in gain switching occurs. For this reason, the top portion of the output voltage signal Vout1 of the preamplifier 200B has larger amplitude amplified with the gain corresponding to the resistance value RF1.
The offset compensator 302 detects the level of the top portion of the signal (top bit) as the detection level of the output voltage signal Vout1. For this reason, the level of the output voltage signal Vout1 corresponding to the resistance value RF2 cannot be detected. In this case, an offset compensation voltage signal Vaoc detected by the offset compensator 302 is different from the normal value.
Note that, in addition to the waveform (alternate long and short dash line) of the offset compensation voltage signal Vaoc, the output voltage signal Vout1 (solid line) of the preamplifier 200 which is the input signal into the offset compensator 302, and the waveform of the detection level (dashed line) are described in the waveform diagram of the offset compensation voltage signal Vaoc shown in FIG. 5. Hence, offset compensation is performed on the output voltage signal Vout2 of the post amplifier 300B according to the level of the top bit. This results in the following problem. The portions of the output voltage signal Vout2 having the level corresponding to the resistance value RF2 cannot have a normal waveform output (dashed line) shown in the waveform diagram of the output voltage signal Vout2 of the post amplifier 300B shown in FIG. 5, and thereby a deformed and deteriorated waveform (solid line) is obtained.
The circuit delay time relative to a signal speed becomes longer especially in a high speed operation. For this reason, there is a problem that the influence of the above problem becomes relatively larger.
In other words, conventionally, the high speed offset compensation is required for the purpose of receiving packets of data signals having different receiving levels. Furthermore, the gain of the preamplifier 200B needs to be switched for the purpose of achieving high sensitivity and a wide dynamic range. However, as the operation speed increases, level variations are more likely to occur in a waveform. Accordingly, an offset compensation error is caused in the offset compensator 302 of the post amplifier 300B. This leads to a problem that the waveform and the transmission characteristics deteriorate.
As described, in the conventional receiving circuit capable of burst transmission, the gain of the preamplifier is switched to widen the dynamic range of the input signal. In the conventional receiving circuit, however, level variations occur due to delay time of the switching control circuit with the high speed operation. This leads to the level detection error of the post amplifier at the subsequent stage, and improper offset correction. As a result, the problem of distortion of waveform and deterioration in sensitivity is caused.